It would seem as if "serverfault.com" should read as "serverfault" like all other sister site links in the footer. Likewise the same change should be applied to both "stackoverflow.com" and "superuser.com".

1 Answer
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I would imagine the reason why Stack Overflow, Server Fault & Super User all use .com in their links is because they have their own domains and are part of the trilogy. The founders of Stack Exchange if you like.

The rest of the sister sites are all Stack Exchange 2.0 sites and all "live" on the *.stackexchange.com domain.